1. Field of the Invention
The invention relates to an implantable medical apparatus for stimulating the heart of a patient, which generates a measurement signal dependent upon impedance in the area around the heart from which a heart rate signal, a stroke volume signal, a respiratory rate signal and a tidal volume signal are derived, and wherein the rate at which the stimulation pulse generator provides stimulation pulses is controlled based on these signals.
2. Description of the Prior Art
A pacemaker for stimulation of a heart is disclosed in German OS 37 32 640, corresponding to U.S. Pat. No. 4,884,576, wherein the stimulation is associated with the pacemaker wearer's level of physical activity. The pacemaker has a stimulation pulse generator which generates and delivers stimulation pulses with a waveform and at a rate governed by a control apparatus. The pacemaker also includes an impedance meter, which records the impedance variations arising in the body primarily because of cardiac and pulmonary activity, to sense physical activity. The impedance signal emitted by the impedance meter is split by two signal filters into a low-frequency signal portion corresponding to pulmonary tidal volume and respiratory rate and a high-frequency signal portion corresponding to cardiac stroke volume and heart rate. The respective signal portions are then analyzed in separate signal analyzers connected to the control apparatus.
The stimulation rate of this known pacemaker is determined by the relation between heart rate and respiratory rate, represented by a single characteristic. The control apparatus can then, on the basis of the other parameters, displace the characteristic so the stimulation rate at a given respiratory rate value becomes dependent on the other parameters. For example, the characteristic can be displaced so the stimulation rate increases when stroke volume increases, even if respiratory rate does not change.
The objective of controlling heart rate is to control the blood's oxygenation. Increased physical activity increases the need for oxygen delivery. In a healthy person, this is attained by an increase in pulmonary minute volume, which is a calculated average value for the volume of air inspired per minute, and cardiac output, which is a calculated average measure of the volume of blood ejected by the heart per minute. Pulmonary minute volume is thus the product of respiratory rate times tidal volume, and cardiac output is the product of heart rate times stroke volume.
However, air in the lungs is not replaced as effectively at a fast respiratory rate as at a slow respiratory rate. As a result, a dead-space volume develops in the lungs, thereby reducing oxygen supplies in the lungs and, accordingly, oxygenation of the blood. In a similar manner, a fast heart rate also reduces oxygenation of the blood, since blood then passes through the lungs more rapidly than at a slower heart rate.